1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system for applying a gradient magnetic field and an RF pulse to an object to be examined which is placed in a static magnetic field so as to excite magnetic resonance at a specific portion of the object, and acquiring magnetic resonance (MR) echo signals excited by the magnetic resonance, thereby imaging the specific portion by a predetermined image reconstruction method using data based on the acquired MR echo signals and, more particularly, to an improvement of an MR echo signal data acquisition sequence for preventing an artifact caused by a chemical shift, increasing an S/N (Signal-to-Noise ratio), and images separated by the chemical shift.
2. Description of the Related Art
In a general medical MRI system, a gradient magnetic field and an RF pulse are applied to an object to be examined which is placed in a static magnetic field in accordance with a predetermined sequence for magnetic resonance excitation/MR data acquisition so as to cause an MR phenomenon at a specific portion of the object, and an MR signal excited by the MR phenomenon is detected. In addition, according to the system, data processing for imaging which includes image reconstruction is performed for MR data acquired in this manner so as to image anatomical information or quality information of the specific portion of the object.
An MRI system of this type generally comprises a static magnetic field generator, X-axis, Y-axis, and Z- axis gradient magnetic field generators, an RF transmitter, and an RF receiver. The X-axis, Y-axis, and Z-axis gradient magnetic field generators and the RF transmitter are driven in accordance with a predetermined sequence so as to generate X-axis, Y-axis, and Z-axis gradient magnetic fields Gx, Gy, and Gz and an RF pulse in accordance with a predetermined sequence pattern. As a result, magnetic resonance is excited to generate an MR signal, and the MR signal is received by the receiver. Predetermined image processing including image reconstruction processing such as the two-dimensional Fourier transform is performed for the received MR data. In this manner, a tomographic image of a certain slice portion of an object to be examined is generated and displayed on a monitor.
In the sequence for magnetic resonance excitation/ MR data acquisition, the X-axis, Y-axis, and Z-axis gradient magnetic fields Gx, Gy, and Gz are respectively used as, e.g., a read gradient magnetic field Gr, an encode gradient magnetic field Ge, and a slicing gradient magnetic field Gs.
One of the conventional MRI methods widely used in such a system is an imaging method employing the sequence of the SE method which uses 90.degree.-180.degree. series RF pulses. According to the sequence of the SE method, data acquisition can be performed by a multi-echo sequence in which a plurality of MR echoes are sequentially generated upon one MR excitation and the respective data are sequentially acquired. This SE method is often used for MR data acquisition using the multi-echo sequence.
The sequence of such a conventional SE method will be described below with reference to FIG. 1. FIG. 1 shows a sequence in one encode step.
A slicing gradient magnetic magnetic field Gs and a 90.degree. selective excitation pulse as an RF magnetic field are applied to an object to be examined so as to excite a specific slice of the object by flipping the magnetization vector (to be referred to as "nuclear magnetization" hereinafter) of the nuclear spin of a specific atomic nucleus in the slice through 90.degree.. Thereafter, an encode gradient magnetic field Ge having an amplitude corresponding to the encode step is applied to the object, and a 180.degree. pulse as an RF magnetic field is applied to the object so as to invert the nuclear magnetization, thereby rephasing and refocusing the rotational phase of the nuclear magnetization (which has been dephased and dispersed upon application of the 90.degree. pulse). In addition, a read gradient magnetic field Gr is applied to the object to generate an MR echo signal whose peak appears after a TE time (echo time) from the peak of the 90.degree. pulse. While the read gradient magnetic field Gr is applied to the object, the MR echo signal is acquired.
The above-described sequence is repeated while the amplitude of the encode gradient magnetic field Ge, which is applied between applications of 90.degree. and 180.degree. pulses, is changed by a predetermined value in every encode step. When the second and subsequent multi-echo signal data are to be acquired, an operation of applying a 180.degree. pulse to the object after a time TE/2 from the peak of the immediately preceding echo signal and applying a read gradient magnetic field Gr to the object is repeated, thereby sequentially acquiring echo signals each having a peak appearing after a time TE/2 from the peak of each 180.degree. pulse while the read gradient field Gr is being applied.
In an MRI system of the type described above, in order to prevent the artifact based on chemical shift (i.e., typically chemical shift between water and fat associated with protons (.sup.1 H)), the strength of the I gradient field Gr is increased to satisfy the following inequality (See, Bruce Barker, "Chemical Shift Artifact In Non-Spectroscopic NMR Imaging", Book of Abstracts: Society of Magnetic Resonance in Medicine, 1984, pp. 34-35): EQU .gamma..multidot.Gr.multidot..DELTA.l.gtoreq..delta..multidot..gamma..multi dot.BO
(where .gamma. is the gyromagnetic ratio, .DELTA.l is the resolution, .delta. is the ohemical shift between water and fat, i.e., chemical shift of the fat, and BO is the static field strength).
However, with this method, a frequency band .DELTA.f within one pixel is increased, and a noise component N is increased as follows: ##EQU1##